1. Field of the Invention
The present invention relates to multiphase alternating current (AC) motor driving devices for driving a multiphase AC motor with an inverter circuit, in particular, to a multiphase AC motor driving device for determining the occurrence of failure based on the detection results of the current values of the respective phases.
2. Description of the Related Art
An electric power steering device in a vehicle is provided with an AC motor such as a three-phase brushless motor in order to apply to a steering mechanism a steering aiding force corresponding to a steering torque of a handle. A motor drive unit includes a PWM (Pulse Width Modulation) circuit for generating PWM signals having a duty ratio corresponding to the command value and an inverter circuit including pairs of upper and lower switching devices, which are turned ON/OFF according to the duty ratio of the PWM signal output from the PWM circuit, provided for respective phases. The inverter circuit outputs voltage for the respective phases corresponding to the aforementioned duty ratio based on the ON/OFF operations of the switching devices, to drive the motor with the voltages. The currents in the respective phases of the motor are detected by a current detection resistance connected in series with the switching device, and the detected value is the value of the current actually flowing through the motor.
Japanese Laid-Open Patent Publication No. 2005-143153, No. H6-253585, No. 2003-164159, and No. 2007-189825 described below disclose a multiphase AC motor driving device using the inverter circuit. In the device of Japanese Laid-Open Patent Publication No. 2005-143153, an estimated value of the current flowing through the inverter circuit is calculated based on an application voltage and a resistance value between a power supply and a motor, and an actual current value detected in an actual current detecting section and the estimated current value are compared to detect abnormality in the values of the currents flowing through the respective phases. In the device of Japanese Laid-Open Patent Publication No. H6-253585, using the fact that the sum of the current values of the respective phases of the three-phase AC motor is zero, determination is made as abnormal when the sum of the detected current values of the respective phases is not zero. In the device of Japanese Laid-Open Patent Publication No. 2003-164159, a first current value including a voltage drop value of a current detection resistance is used as a current value of a phase in which duty ratio of a switching device on the lower side in the inverter circuit is greater than or equal to a predetermined value, and a second current value in which the sign of the sum of the voltage drop values of the current detection resistances of the other two phases is reversed is used as a current value of the phase which duty ratio of the switching device on the lower side is smaller than a predetermined value, so that the current detection accuracy can be enhanced by the second current value even if the duty ratio of the device is small and the current cannot be accurately detected In the device of Japanese Laid-Open Patent Publication No. 2007-189825, the phase currents flowing through the reflux diodes connected in parallel to the switching devices are detected during a dead time period in which the upper and lower switching devices of the inverter circuit are both turned OFF to lengthen the sampling period and enable motor control with high accuracy.
In the motor driving device using the inverter circuit as described above, the sum of the currents of the respective phases of the motor is known to be zero, and thus the occurrence of failure of the motor driving device or the motor can be determined by detecting the currents of the respective phases and determining whether the sum is zero as in Japanese Laid-Open Patent Publication No. H6-253585.
In an inverter circuit of turning ON/OFF the switching devices arranged in upper and lower pairs for the respective phases with a PWM signal, the turn-ON time T2 of the lower side switching devices becomes long when the turn-ON time T1 of the upper side switching devices is short (i.e., when the ON duty ratio is small), as shown in FIG. 4, and the turn-ON time T2 of the lower side switching devices becomes short when the turn-ON time T1 of the upper side switching devices is long (i.e., when the ON duty ratio is large), as shown in FIG. 5.
In the case of FIG. 4, the turn-ON time T2 of the lower side switching devices is long, and thus the phase current flowing through the current detection resistance through the switching device can be accurately detected; whereas in the case of FIG. 5, the turn-ON time T2 of the lower side switching device is short, and thus the phase current flowing through the current detection resistance through the switching device becomes difficult to detect accurately. That is, as disclosed in Japanese Laid-Open Patent Publication No. 2003-164159, when the duty ratio of the lower side switching devices becomes smaller than 30%, the switching devices may not be sufficiently turned ON due to obtuseness etc. of the waveform of the gate voltage to be applied to the device, and the phase current cannot be accurately detected. When charging the voltage generated in the current detection resistance of the respective phase to the condenser, sample holding the same, and detecting the phase current, the turn-ON time of the switching device becomes shorter than a time constant of a charging circuit, sample hold is not accurately performed, and the error occurs in the detection of the phase current.
As a result, the sum of the current values of the respective phases detected by the current detection resistances does not become zero, and determination is sometimes made that failure has occurred. The sum of the currents does not become zero because the phase currents cannot be accurately detected since the duty ratio of the lower side switching devices is small, and not because abnormality is found in the motor 4 and the drive circuit, and thus the determination is an erroneous determination.
Thus in the conventional device, determination is erroneously made that failure has occurred when actually failure has not occurred when the duty ratio of the lower side switching devices of the inverter circuit is small.
It is an object of the present invention to provide a multiphase AC motor driving device which does not make an erroneous determination on the occurrence of failure.